Field of the Invention
The present invention is broadly concerned with novel temporary wafer bonding and debonding methods that support a device or other ultrathin layer during thinning, transfer, and/or backside processing, as well as structures resulting from those methods.
Description of the Prior Art
Advances in technology, such as epitaxial layer growth through metalorganic chemical vapor deposition (MOCVD) and dice-before-grind (DBG) processing have created the need for new methods to handle ultrathin materials. Epitaxial layers, such as gallium nitride and aluminum gallium indium phosphide are typically grown 10-μm to 25-μm thick on a fall thickness growth substrate that is typically formed of sapphire, gallium arsenide, or germanium wafer. While this approach is convenient, it is often advantageous to remove the epitaxial layers from the growth substrate. While there are several methods to separate the epitaxial layer from the growth substrate, including diamond wheel grinding, wet chemical etch, and laser lift-off (LLO), methods for handling the separated layer are inadequate. In the case of DBG processing, it is necessary to support the pre-diced wafers as they are thinned, while allowing separation after thinning.
Currently, there are no known robust, cost-effective solutions for the handling of these ultrathin materials, and certainly no known methods for epitaxial layer handling. Some methods do exist for handling thin wafers. In many cases, the thin wafers (usually silicon) are temporarily bonded to a carrier, such as a standard glass or silicon wafer. These wafer pairs are bonded, usually by a polymeric material, which can then be removed by a heat or chemical treatment. However, these processes cannot be applied directly to ultrathin materials, such as epitaxial layers. The materials developed for thin silicon bonding were designed to flow at bonding temperatures of around 180° C. At this temperature, flowing is not acceptable for ultrathin material handling. Thinned wafers that are about 50-μm thick can bend, but are basically rigid, and therefore will remain flat during a thermal cycle that allows the bonding material to flow. Free-standing epitaxial layers are so thin that they lack the rigidity required to be self-supporting. If the bonding material is able to flow, the layer will relieve any stress by forming wrinkles or other distortions. This is especially problematic with heteroepitaxial systems that, due to lattice mismatches and differing CTE from layer to layer, are loaded with stress.
As an additional challenge, separation techniques used to remove epitaxial layers from their growth substrates can be highly traumatic. Laser lift-off can involve extremely high local temperatures, and bulk chemical etch processes can be harsher than processes typically encountered with typical silicon wafer processing. Current methods utilize materials that are difficult to separate from the carrier wafer and are not readily cleanable from the ultrathin epitaxial layer.
The present invention overcomes the problems of the prior art by decoupling the requirements of bonding and support by using a multi-layer or multi-functional system.